[4] Auscultation is still generally considered to be the gold standard of accuracy for non-invasive blood pressure readings in clinic.

[8] Blood pressure is influenced by cardiac output, systemic vascular resistance, blood volume and arterial stiffness, and varies depending on person's situation, emotional state, activity and relative health or disease state.

In the short term, blood pressure is regulated by baroreceptors, which act via the brain to influence the nervous and the endocrine systems.

[17] Blood pressure varies over longer time periods (months to years) and this variability predicts adverse outcomes.

[31] In adults in most societies, systolic blood pressure tends to rise from early adulthood onward, up to at least age 70;[32][33] diastolic pressure tends to begin to rise at the same time but start to fall earlier in mid-life, approximately age 55.

[34] An age-related rise in blood pressure is not considered healthy and is not observed in some isolated unacculturated communities.

Higher pressures increase heart workload and progression of unhealthy tissue growth (atheroma) that develops within the walls of arteries.

[50] For people with high blood pressure, higher heart rate variability (HRV) is a risk factor for atrial fibrillation.

In a study of people with heart valve regurgitation that compared measurements two weeks apart for each person, there was an increased severity of aortic and mitral regurgitation when diastolic blood pressure increased, whereas when diastolic blood pressure decreased, there was a decreased severity.

[59] Causes of low arterial pressure include sepsis, hypovolemia, bleeding, cardiogenic shock, reflex syncope, hormonal abnormalities such as Addison's disease, eating disorders – particularly anorexia nervosa and bulimia.

[60] A large fall in blood pressure upon standing (typically a systolic/diastolic blood pressure decrease of >20/10 mmHg) is termed orthostatic hypotension (postural hypotension) and represents a failure of the body to compensate for the effect of gravity on the circulation.

Standing results in an increased hydrostatic pressure in the blood vessels of the lower limbs.

The consequent distension of the veins below the diaphragm (venous pooling) causes ~500 ml of blood to be relocated from the chest and upper body.

Normally this is compensated for by multiple mechanisms, including activation of the autonomic nervous system which increases heart rate, myocardial contractility and systemic arterial vasoconstriction to preserve blood pressure and elicits venous vasoconstriction to decrease venous compliance.

[61] A similar effect is observed following the experience of excessive gravitational forces (G-loading), such as routinely experienced by aerobatic or combat pilots 'pulling Gs' where the extreme hydrostatic pressures exceed the ability of the body's compensatory mechanisms.

Variation in blood pressure that is significantly greater than the norm is known as labile hypertension and is associated with increased risk of cardiovascular disease[63] brain small vessel disease,[64] and dementia[65] independent of the average blood pressure level.

[70] These data have prompted discussion of whether excessive variation in blood pressure should be treated, even among normotensive older adults.

[73][unreliable medical source] The blood pressure in the circulation is principally due to the pumping action of the heart.

Mean blood pressure drops over the whole circulation, although most of the fall occurs along the small arteries and arterioles.

This has been proposed as an explanation of the relationship between high dietary salt intake and increased blood pressure; however, responses to increased dietary sodium intake vary between individuals and are highly dependent on autonomic nervous system responses and the renin–angiotensin system,[79][80][81] changes in plasma osmolarity may also be important.

[83] In simple terms, systemic vascular resistance is mainly determined by the caliber of small arteries and arterioles.

[88] In practice, each individual's autonomic nervous system and other systems regulating blood pressure, notably the kidney,[89] respond to and regulate all these factors so that, although the above issues are important, they rarely act in isolation and the actual arterial pressure response of a given individual can vary widely in the short and long term.

The increased risk also exists even in cases in which diastolic pressure decreases over time while systolic remains steady.

The study authors also noted that, while risks of cardiovascular end points do increase with higher systolic pressures, at any given systolic blood pressure the risk of major cardiovascular end points increases, rather than decreases, with lower diastolic levels.

[93] Pulse pressure can both widen or narrow in people with sepsis depending on the degree of hemodynamic compromise.

A pulse pressure of over 70 mmHg in sepsis is correlated with an increased chance of survival and a more positive response to IV fluids.

[98][99] The endogenous, homeostatic regulation of arterial pressure is not completely understood, but the following mechanisms of regulating arterial pressure have been well-characterized: These different mechanisms are not necessarily independent of each other, as indicated by the link between the RAS and aldosterone release.

Generally, the baroreceptor reflex is not targeted in hypertension because if blocked, individuals may experience orthostatic hypotension and fainting.

[109] The giraffe has a distinctly high arterial pressure of about 190 mm Hg, enabling blood perfusion through the 2 metres (6 ft 7 in)-long neck to the head.

[112][113] As in humans, blood pressure in animals differs by age, sex, time of day, and environmental circumstances:[114][115] measurements made in laboratories or under anesthesia may not be representative of values under free-living conditions.